It appears on DataWriter/AddOn.php, CodeEventListener, CronEntry, Page and UserField

And well, of course the phrase needs to be updated, that would reduce the hunting time down on misbehaving add-ons

Today, whenever I try to install an add-on that has a missing class, it says "Please enter a valid callback method" and leaves me hunting down whether it meant the installer, an add-on, a cron job, etc.

And this usually happens whenever it works perfectly on non-case-sensitive OS and then the add-on is installed on a case-sensitive OS and suddenly it cannot find one file.